a) Field of the Invention
This invention relates to a process for the production of an acrylonitrile (co)polymer without occurrence of adhesion of the (co)polymer on an inner wall of a reaction tank having a vapor phase and a liquid phase upon (co)polymerization of a monomer material, which includes an acrylonitrile monomer as an essential component, in the reaction tank and also to a polymerization reaction tank suitable for use in the practice of the production process.
b) Description of the Related Art
Production of a (co)polymer from a monomer material, which comprises at least an acrylonitrile monomer as an essential component, has been conventionally conducted by emulsion polymerization or suspension polymerization. These polymerization processes are however accompanied by serious problems from the standpoints of the global environment and product quality as they result in production of a great deal of waste water and mixing of an emulsifying or a suspending agent in products. There is hence an outstanding important demand for the development of a bulk or solution polymerization process.
However, neither bulk nor solution polymerization permits removal of heat by water as a medium. Use of a large reaction tank therefore raises a problem, that is, how to remove heat of polymerization reaction.
Upon (co)polymerization of a monomer material including at least an acrylonitrile monomer as an essential component, the (co)polymerization is generally conducted with a reaction tank fully filled up with a reaction mixture or in the presence of a vapor phase in the reaction tank.
When the (co)polymerization is conducted with the reaction tank fully filled with the reaction mixture, removal of heat has to be effected primarily relying upon conduction of heat through a tank wall so that, when the reaction tank becomes large, difficulty is encountered in removing heat. To achieve the above-mentioned removal of heat, it has therefore been the conventional practice to arrange coiled tubes of complex configurations within the reaction tank or to externally arrange a heat exchanger so that external recirculation of the polymerization mixture is performed to recirculate and cool it through the heat exchanger.
However, the (co)polymerization of said monomer material including at least one acrylonitrile monomer as an essential component is prone to adhesion of a gelled polymer on an inner surface of the reaction tank, whereby the use of the coil of the complex configurations or the heat exchanger for the provision of a sufficient heat-conducting area is disadvantageous in this respect. In particular, use of a reaction tank of a large capacity requires a greater area for the conduction of heat and more complex production facilities, thereby making the troublesome occurrence of a gelled polymer more serious.
When the capacity of a reaction tank is 1 m.sup.3 or greater, especially over 3 m.sup.3, it is advantageous to provide a vapor phase in an upper part so that heat of polymerization is controlled by allowing vapor to evaporate from a polymerization mixture, in other words, by so-called latent heat of evaporation. As the heat occurring through the polymerization is removed by the latent heat of evaporation, the reaction tank may be of a relatively simple construction. This easy removal of heat has also led to the merit that the size of the reaction tank can be enlarged.
However, in the case of inclusion of acrylonitrile as a vinyl monomer, especially copolymerization of styrene and acrylonitrile in the above-mentioned control method making use of latent heat of vaporization, a gaseous monomer mixture composed predominantly of acrylonitrile exists in a vapor phase, thereby involving another problem that a scale of a (co)polymer rich in acrylonitrile tends to deposit on a tank wall corresponding to the vapor phase. Gradually, this acrylonitile-rich (co)polymer scale is gelled and is rendered insoluble in a solvent. Portions of the scale dropping from the tank wall corresponding to the vapor phase into the polymerization mixture in the tank results in the serious problem that a resin obtained as a product is rendered impure. In particular, when the resulting copolymer is used as a film or sheet, inclusion of a polymer from the tank wall corresponding to the vapor phase has been a cause for formation of fish eyes. To remove a gel adhered on the reaction tank, the reactor has to be stopped frequently and a great deal of labor is needed. The formation of such a scale has created a serious problem for industrial production.
As a method for the prevention of formation of a polymer gel in a vapor phase, it has been proposed to coat a tank wall corresponding to the vapor phase with an anti-scaling agent. This method is however applicable only when a styrene and/or acrylonitrile monomer(s) is polymerized in an aqueous medium. In a process other than this, for example, in bulk polymerization or solution polymerization, the above-proposed method however involves the drawback that the anti-scaling agent is washed away by the monomer(s) and is not usable where a long-term continuous operation is desired.
Further, it has also been proposed in Japanese Patent Publication No. 15641/1981 (Mitsubishi Rayon) and U.S. Pat. No. 4,677,171 (Cosden Technology) to maintain the interior of a reaction tank at such a pressure so as to prevent evaporation of a polymerization mixture and to condense a gas over the polymerization mixture by a cooling device arranged inside or outside the reaction tank for the prevention of adhesion of a polymer on an inner wall of the reaction tank and the cooling device and also for the removal of heat of polymerization. However, this method requires pressurization of the interior of the reaction tank with an inert gas to maintain the polymerization mixture under a pressure preventing its evaporation, and raises a new problem that, due to the cooling of the vaporized polymerization mixture with the inert gas, the efficiency of heat conduction is deteriorated and the cooling device has to be very large.
It has also been proposed to prevent adhesion of a polymer inside a reaction tank by allowing a mercaptan of a low boiling point to exist in a vapor phase. This method however imposes limitations on operation conditions such as reaction temperature and reaction pressure. In addition, the mercaptan acts as a chain transfer agent and affects the reaction, for example, lowers the molecular weight of a polymerization product, resulting in a problem in the quality control of the product.